Modulating system



Peasant; ii, 15251 PATENT. OFFICE.

EDWIN E. SMYTHE, OF EVANSTON, ILLINOIS, PANY, INCORPORATED, OF NEW YORK, N.

ASSIGNOR T WESTERN ELECTRIC cou- Y., a CORPORATION on NEW You,

MODULATING SYSTEM.

Application filed May 24,

This invention relates to a. system for producing a carrier "wave modulated in ac cordance with signals and more particularly to an arrangement utilizing the principles of constant current systems for producing modulated components while suppressing the unmodulated carrier component.

It has heretofore been proposed to provide a system including balanced variable impedance devices associated with a carrier wave source comprising balanced space discharge devices having their input and output circuits inductively coupled in feedback relation" by means of a frequency determining circuit common to both discharge devices, the variable impedance devices and the discharge devices, being supplied w1th energy from a substantially constant current source.

In such a system the signal current is supplied through a common circuit to control the operation of the variable impedance devices which in turn cause space current, varying in accordance "with the signals, to be supplied to the discharge devices included in the oscillators, whereby signal modulated components, but no unmodulated component of carrier frequency, are supplied to a transmission circuit associated with the devices.

In such a system the coupling indnctances included in the associated circuits of the discharge devices oifer ap reciable impedance to the flow of the modulated components, with the result that, for any given set of discharge devices operating under redetermined conditions, the energy supp ied to the transmission circuit is limited.

The present invention provides a system, similar to that described above for supplying carrier modulated components to a transmission circuit, while suppressing the unmodulated component. This system' is adapted to operate at high efliciency.

In accordance with this invention a car- -rier wave is supplied by a separate source through a common circuit to the input circuit of the discharge devices each having an alternating current output circuit which offers relatively low impedance to the modu- 1924. Serial No. 715,519.

carrier suppression constant current modulating system of relatively high efiiciency.

For a complete understanding of the invention reference may be made to the following descriptioff'readdn gonjunction with the attached drawing, in which Fi' ureul shows a modulating system involving this" invention and Fig. 2 shows an arrangement, which may be substituted for that on-the right of section line A-A of Fig. 1, whereby the invention may be used at the terminal station of a multiplex carrier system in which the carrier waves for the different channels are harmonics of a base frequency wave supplied from a master control station.

In the drawing there is shown a modulating system in which a signaling or modulating current is supplied by a circuit 1. For purposes of illustration, the circuit 1 is shown as comprising a microphone 2, a battery'3 and a sectionalized coil 4. Obviously the circuit 1. may be a tele hone line, a telegraph line, or other suita le apparatus adapted to supply current for modulating a earner wave.

The low frequency signal current is supplied to the input circuit of the space discharge devices 5 and 6 by means of the coils 7 inductively associated with the coil 4. The input circuit of each of the devices 5 and 6 extends from its cathode through the coil 7 shunted by a condenser 8, to its imp'edance control electrode, vand across the space within the device to its cathode.

Heating current is supplied to the oathodes of the devices 5 and 6 by the source 9 and space current is supplied thereto by the source 10. The space current path for each device may be traced from the positive terminal of the source 10, choke coil 11, the anode or plate of the device across the space within it to its cathode, and back to the negative terminal'of the source 10.

Variable impedance devices 12 and 13, for example, three-electrode space discharge devices, are connected in balanced relation and are associated with the devices 5 and 6.

Space current is supplied to each of the devices 12 and 13 over the following path; from the positive terminal of the source 10, choke coil 11, coil 14, anode of the discharge device, across its space path t'o'the cathode, through the conductor 15 to the negative terminal of thesource 10.

The alternating output current path for the discharge devices 5 and 6 extends from the anodes through the coil 14, to the anodes of the devices 12 and 13 across the space paths within them to their cathodes, conductor 15 to the cathodes of devices 5 and 6, across the space paths thereof to their anodes.

Carrier current produced by a source 16 is supplied to the input circuits of the discharge devices 12 and 13. The coil 18 is inductively associated with the coils 19, which are included in the respective input circuits of the devices 12 and 13. I

The input circuit of the device lamaybe" traced asfollows from its cathode through the conductor 20 upper coil 19, its impedance control "electrode and across the space path within it to its cathode. Similarly the input circuit of device 13' extends from its cathode, conductor 20, lower coil 19, control electrode of tube 13 and across the space path to its cathode. Heating current is suplied .to the cathodes of devices 12 and 13 y the source 25.

The alternating current path for the devices 12 and 13 may be traced from their anodes through the condensers 21 and coil 22 to the cathodes across their space paths to their anodes. The loadcircuit 23 is coupled to the alternating current output circuit of the devices 12 and 13 by the coil 24 inductively associated with the coil 22.

The devices 5 and 6 serve to amplify the modulating current supplied by the circuit 1 and the control electrodes of these devices are supplied with the desired polarizing potentials by the sources 26. In a similar manner polarizing potentials are supplied to the. control electrodes of the discharge devices 12- and 13 by the sources 27 Choke coils 14 are of high impedance to carrier frequency waves but of low impedance for signal frequency currents and are adapted to prevent dissipation of the carcathode circuits of the devices 5 and 6.. The

coil sections 7 included in each grid circuit are of equal impedance for thesignal currents.

During operation the impedances of the devices 5 and 6 will be varied by the signal currents in the opposite sense, that is, when one device is made of higher impedance the other device will be'of lower impedance and hence'the space current supplied to the variable impedance devices 12 and 13 will vary at a signal frequency rate. Therefore, when one device 12 ispassing carrier waves of higher than average amplitude the other device 13 will be passing carrier waves of lower than average amplitude.

When no signaling currents are applied to the input circuits of the devices 5 and 6 the amplitude. of the carrier frequency wave in the load circuit 23 will be small or zero, because the output circuits of the devices 12 and 13 are associated by a common coupling to the load circuit and are connected in opposite relation thereto. The coils 19 are of equal impedance for carrier frequency waves and the batteries '27 impress such potentials and 13 as to make the devices 12 and 13 of equal impedance. The condensers 21 are of low impedance for carrier frequency waves and are of the same capacity.

This system operates, analogously to the manner described in U. S. Patent 1,343,307, June 15, 1920, to Carson, to suppress the unmodulated carrier component while the carrier modulated waves of side band fre quencies produced by the devices 12 and 13, are impressed upon the load circuit in series, approximately 180 out-of-phase with each other.

The alternating current output circuits of the devices 12 and 13 are of relatively loW impedance for the side band frequency components and hence the amplitudes of these components flowing through the coil 22 will be large, with the result that signal modulated waves of materially increased amplitudes, as compared with those of previously known systems, will be transmitted to the load circuit.

In Fig, 2 there is shown a. source 16 of fundamental frequency situated at a master control station M. 6.1 The wave of fundamental frequency is transmitted over a line 28 and impressed upon a harmonic generator 29, which "may be of the type disclosed in U. S. Patent 1,446,752, February 27, 1923, to Kendall.

lot

One of the harmonics to be used for one outgoing channel "at the terminal station is selected by the circuit 17 including the coil 18 and condenser 30. The coil 18, as in the arrangement described above, is inductively associated with the coils 19 to supply a carrier wave to the input circuits of the variable impedance devices 12 and 13.

A second tuned circuit 17 is shown associated with the output circuit. of the harmonic generator 29. This circuit is resonant. to a different harmonic frequency from that selected by the circuit 17 and its coil 18' is impressed upon the harmonic generator 29 and the harmonic components proper to the,

channel or channels of the terminal station under consideration would be selected by the circuits 17 and 17'.

From the preceding description, it will be seen that the'present invention provides a modulating system comprising two electrically symmetrical circuits each consisting of an amplifier and a modulator supplied with energy in parallel from a substantially constant current source in which the low frequency signal current circuit is oppositely coupled to the input circuits of the amplifiers, the carrier wave source is coupled oppositely to the input circuits of the modulators and the transmission circuit is coupled to a common portion of the output circuits of the modulators, with the result that only the side band components of the modulated Wave are supplied to the transmission circuit. The invention therefore provides a system in which the advantages of constant current modulation, carrier suppressiom'and remote control of the carrier frequency are combined.

' While certain circuits and specific details have been described for the purpose of clearly and completely disclosing the nature of the invention, it is to be understood that this invention is not limited to the specific arrangement described, but only by the scope of the attached claims.

What is claimed is: p 1. A modulating system comprising a plurality of constant current systems, each including a signal frequency discharge device and a variable impedance device, a source of signal waves coupled in common to the input circuits of the signal frequency devices, carrier frequency input and output circuits for said variable impedance devices, a source of carrier waves cou led to homologous circuits of said variab e impedance devices, a

transmission circuit, and means for coupling other homologous circuits of said variable impedance devices to said transmission circuit.

2. A modulating system comprising a pluralit of constant current systems, each including a signal frequency discharge device and a variable impedance device, a source of signal waves coupled in common to the input circuits of the signal frequency devices, carrier frequency input and output circuits for said variable impedance devices, a source of carrier waves coupled to the input circuits of said variable impedance devices, a

transmission circuit, and means for coupling the output circuits of said variable impedance devices to said transmission circuit.

3. A carrier transmission circuit comprising two electrically symmetrical circuits each comprisin an amplifier and a modulator, each having an input and an output circuit, a common signal frequency input source for said circuits, associated with the input circuits of said amplifiers, a common carrierv frequency input source for said circuits associated with said modulators, means for connecting the output circuits of said amplifiers respectively with the output circuits of said modulators to produce modulation of the carrier frequency waves in accordance with the signal frequency waves, and means for coupling the output circuits of the modulators differentially to a transmission circuit. v

4. A modulating system comprising a plurality of constant current systems each comprising a signal frequency amplifier and a variable impedance discharge device, said amplifiers havin a common low frequency input circuit an an output circuit, said variable impedance devices having a common carrier frequency input circuit and a common carrier frequency output circuit, means operatively associatingthe output circuit of said amplifiers with the output circuit of said impedance devices for effecting modulation of the carrier frequency waves by the amplified signal frequency waves, a transmission circuit, and means for coupling said carrier frequency output circuit to said transmission circuit.

5. A modulating system comprising two electrically symmetrical constant current systems each comprising a signal frequency vamplifier and a variable impedance device in parallel with respect to a common source of current and a speech frequency choke coil, a common speech frequency input circuit oppositely connected to said amplifiers, a common carrier frequency input circuit op positely connected to said variable impedance devices and a common high frequency output circuit for said variable impedance devices.

=6. A carrier signaling system compr sing a master control station, a source at said stato said modulators for supplying thereto :1 tion for supplying the carrier Waves used in carrier wave received from said master st'a- 10 said system, a terminal station provided with tion and means for coupling the modulators a modulating system consisting of two elecoppositely to a transmission circuit.

trically symmetrical circuits each including 7 In Witness whereof, I hereunto subscribe an amplifier and a modulator, a signal fremy name this 21st day of May, A. D. 1924. quency circuit oppositely coupled to said amplifiers, a circuit oppositely coupled EDWIN H. SMYTHE. 

